Reproduction of color involves the creation of an accurate apparent color match between original and reproduction. Color originals may be, for example, pictorial slides, which are analog in nature and have a very wide color gamut, wider than in typical reproduction, such as offset printing. In the age of digital information most of the reproduction process is performed digitally. The original slide is scanned to obtain a file containing the color data in terms of RGB values. The file is then converted to CMYK separations, and afterwards plates are created and installed on a press for print. To obtain color consistency, proofs are performed and examined in various stages of the process to ensure that each step is color-consistent with its previous step.
Accurate reproduction of color is very important for printed matter. Typically, in order to achieve an accurate color match, a “hard proof” may be printed on paper and sent to the customer and/or designer for approval. Upon approval, the proof is delivered to the printing shop, where the printer working on the press machine adjusts the press machine until the printed sheets match the hard proof.
For certain applications, most notably in the packaging industry, there is a need for special colors, many of which are out of the gamut of CMYK process inks. Therefore, standard CMYK hard proofers are unsuitable for such proofing.
The manual procedure of proofing limits the advantages of digital workflow. The need for an accurate digital “soft proof” on an electronic display is clear. Currently available “soft proofing” devices are intended to enable designers and pre-press personnel to view works on a display of a computational device such as a personal computer or workstation. Such devices may be based on Cathode Ray Tubes (CRT) or Liquid Crystal Devices (LCD). The final product, however, is an image printed on paper. Currently, soft proofing devices do not overcome inherent deficiencies of digital print proofing, and in particular do not provide good color match, in the sense that they cannot accurately replicate the colors electronically as they would appear on the printed material. This is a serious drawback, as many printed works are now transferred digitally from design to printed material over a network, and any intermediate procedure that must involve printing onto a physical substrate, prior to the final printing step, significantly reduces the efficiency of the printing process.
It should be noted that, for various reasons, CRT color displays generally do not provide an accurate color match to a printed image. As shown in FIG. 1, the color gamut reproduced by a printing press, e.g., an offset printer, is different from that of CRT displays, e.g., there are non-overlapping regions in the printed gamut relative to the CRT gamut and vice versa. Thus, the colors that can be displayed by a CRT monitor do not overlap the colors that can be produced by printing methods. For example, FIG. 1 illustrates the color gamut of offset printing under D50 daylight illumination as compared with the color gamut that can be displayed on a typical CRT monitor. The CRT monitor cannot reproduce the yellow colors, e.g., in a vicinity of color coordinates (x=0.45,y=0.45), nor can the CRT monitor reproduce a wide cyan-green spectrum, e.g., in a region between color coordinates (x=0.2,y=0.2) and (x=0.3,y=0.5). It is evident that a CRT monitor cannot reproduce certain printing colors that have color coordinates outside the CRT gamut. At least part of the limitation of the CRT gamut is associated with the physical properties of the CRT screen, e.g., the emitted spectrum from the red, green and blue phosphors of the CRT. Furthermore, in many cases printers use special colors (e.g., Coca-Cola Red or other trade mark colors like Pantone, Toyo BS, etc.), many of which are outside the gamut of the CRT and the offset CMYK process inks. These colors cannot be matched by CRT monitors, nor can they be matched by a “hard” proofer designed to match process CMYK inks.
A further problem with soft proofing on CRT monitors is that the colors of the inks (and color combinations of such inks) are different from those reproduced by the RGB phosphors of CRT monitors, and therefore a special transformation from CMYK values to corresponding ROB values may be required in order to reach colors closer to an apparent calorimetric match. Such transformation is the basis of existing methods of color matching in general, and soft proofing on display methods in particular. Such methods are based on mapping of the color space of an output device (e.g., printing press, display) onto a device-independent color space, such as L*a*b*, as defined by Commission Iternationale De L'éclairage (CIE). Using this mapping, a multi-dimensional transformation from the RGB space of the display into the L*a*b* space may be performed. Then, another transformation from the L*a*b* space onto the CMYK space of the printing press may be performed. These transformations, known as profiles, are performed on the data file containing the work, before printing, by a color management system. The International Color Consortium (ICC) has standardized this method for color matching.
It is noted that the profiling process described above maps the colors created by CMYK inks printed on a certain substrate and viewed under certain light conditions onto a color space of the RGB phosphors. The spectra of the light reflected off the CMYK inks depends on the lighting conditions, e.g., the spectrum of white light which illuminates the paper of the printed material, and on the reflectivity of the paper. Therefore, different profiles may be required for each combination of paper type and/or ink type and/or illumination conditions, resulting in a cumbersome profiling process.
Furthermore, CRT monitors may not have adequate color consistency and stability, for a number of reasons. First, the electronic circuitry that drives the electron beam is not sufficiently stable, resulting in changes in the brightness of the light emitted from the phosphors. Furthermore, the ratio between the brightness of the Red, Green and Blue (RGB) light may also change, resulting in color variations for a given RGB input value. CRT displays are also highly influenced by external conditions such as magnetic fields. The presence of even slightly magnetized materials near a CRT monitor (such as a loudspeaker, motor, etc) will cause color shifts that are beyond the acceptable level in proofing applications. Thus, a CRT for proofing applications should be used in a highly controllable environment. This dictates the use of specially calibrated and electronically stabilized monitors, such as the Reference and Personal Calibrator™, available from Barco, Kortrijk, Belgium. Furthermore, the exchangeability of CRT monitors is very limited, because the phosphors have tolerances in their emission spectra, resulting in different colors for different CRT units. In addition the phosphors decay and fade over time. All these phenomena require continuing calibration and profiling of the CRT itself.
Moreover, even if the CRTs could be made more stable, they are typically unsuitable for color reproduction. Color accuracy is highly dependent on ambient light conditions. Small amounts of ambient light, reflected from the CRT screen, are added to the light originating from the phosphors, altering the overall appearance of the displayed picture. This effect is very pronounced when viewing relatively dark colors, but even brighter colors are vulnerable to this effect. Since the brightness of the CRT used as computer monitors is typically relatively low, the level of ambient light in a normal working environment is sufficiently high to cause unacceptable color shifts. Thus, the use of CRT for proofing purposes dictates the use of a controlled environment, e.g. a relatively dark room. Further, the high reflectance of “shadow mask” technologies used by many CRT monitors exasperates the color variation problems of existing displays.
The match between images depends also on their level of brightness. As discussed above, the brightness of normal CRT used as computer monitors is relatively low. Enhancement of the brightness of CRT based computer monitors is limited, because the emission of the light from the phosphors is associated with a harmful X-ray radiation created by the deceleration of electrons impinging on the screen. In proofing applications, however, hard prints are typically viewed under a much higher light intensity to maximize image brightness. To compensate for this difference in lighting conditions requirement, attempts have been made to account for different levels of illumination of the print and the CRT, by including perceptual models in the mathematical transformation of the CMYK data to the RGB input. Unfortunately, no such transformation has produced satisfactory results, partly because the transformation correction depends on human perception, which does not have an established mathematical model. Therefore, it is practically impossible to compare a printed sample to a soft proof viewed on a CRT under the same ambient illumination level.
As described above, the RGB spectra reproduced by CRT phosphors is very different from that of color inks and their overlaps. Moreover, in viewing the subtractive color combinations produced by color inks, the number of elementary colors integrated by the eye is larger than that of the standard RGB system. Certain colorimetric match to “in-gamut” colors, as described above, may be possible; however, even if good colorimetric match between print and monitor may be achieved for one observer, such a match is not guaranteed for another observer. This is due to the fact that color is a psychophysical phenomenon, which involves the spectral input to the eye, the optics of the eye, and a perceptual process. Different individuals are likely to differ in their color perception, due to variations in the eye physiology. The significant spectral discrepancy between the CRT phosphors and the printed ink elementary colors often results in situations where a match between CRT and print may be reasonable for one observer, but not acceptable for another observer. Due to this and other deficiencies, a CRT monitor cannot be used as an accurate device for color communication.
Many attempts have been made in the past to adapt CRT displays for soft proofing using calibrated monitors and ICC profiling, e.g., Apple ColorSync™ from Apple Computer Inc., CA, USA, Barco Calibrator™ from Barco, Kortrijk, Belgium, Virtual Matchprint™ from Kodak Polychrome Graphics (KPG), etc. However, CRT based soft proofing has not gained sufficient ground in the industry, mainly due to the deficiencies discussed above.
Other display technologies, e.g., LCD displays of laptop and desktop computers, suffer from some of the problems discussed above, in varying degrees. Furthermore, the color gamut of most flat-panel LCD displays is smaller than that of CRT and, therefore, such displays cover even a smaller fraction of the printed color compared to CRT displays. Additionally, LCD displays have a high variation of color and brightness as a function of viewing angle, whereby a slight change in the viewing angle of the observer may result in significant changes in color.